one way anova overall significant, but dunnett post hoc has no single significant group?
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Hi hope someone can really help, this confuse me alot
I would like to compare the mean for 3 test groups to a control group fro different parameters (RBC, ALT, ALP....), so i ran a oneway ANOVA and then i conduct the post hoc using Dunnett's test..then i observed someting confusing me, whereby There were parameter whereby when ANOVA overall P value was significant, however dunnett test gave no no single group with significant value, how possible? Also, there were situations where overall ANOVA test was not significant, but posthoc (which i included sometime for test run) show some test groups with significant P value as compared to contorl... Wish someone came across with these can help me, many thanks Note: the number of subject in each group might differ, for example, each group can have 3/4 animals and is not throughouhly fixed |
Re: one way anova overall significant, but dunnett post hoc has no single significant group?
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The ANOVA is sometimes referred to as the "omnibus" test as it is testing the null hypothesis against a broad spectrum of alternate hypotheses. I suspect that the initial ANOVA result is picking up something not addressed by the specific Dunnett contrasts, something like the average of the three groups being different from the control. Or perhaps the average of two groups being different than another group...
wbw William B. Ware, Ph.D. McMichael Professor of Education 2011-2013 Educational Psychology, Measurement, and Evaluation CB #3500 - 118 Peabody Hall University of North Carolina at Chapel Hill Chapel Hill, NC 27599-3500 Office: (919)-962-2511 Fax: (919)-962-1533 Office: 118 Peabody Hall EMAIL: [hidden email] <mailto:[hidden email]> Adjunct Professor, School of Social Work Academy of Distinguished Teaching Scholars at UNC-Chapel Hill -----Original Message----- From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of chengfoh Sent: Thursday, October 18, 2012 6:19 AM To: [hidden email] Subject: one way anova overall significant, but dunnett post hoc has no single significant group? Hi hope someone can really help, this confuse me alot I would like to compare the mean for 3 test groups to a control group fro different parameters (RBC, ALT, ALP....), so i ran a oneway ANOVA and then i conduct the post hoc using Dunnett's test..then i observed someting confusing me, whereby There were parameter whereby when ANOVA overall P value was significant, however dunnett test gave no no single group with significant value, how possible? Also, there were situations where overall ANOVA test was not significant, but posthoc (which i included sometime for test run) show some test groups with significant P value as compared to contorl... Wish someone came across with these can help me, many thanks Note: the number of subject in each group might differ, for example, each group can have 3/4 animals and is not throughouhly fixed -- View this message in context: http://spssx-discussion.1045642.n5.nabble.com/one-way-anova-overall-significant-but-dunnett-post-hoc-has-no-single-significant-group-tp5715709.html Sent from the SPSSX Discussion mailing list archive at Nabble.com. ===================== To manage your subscription to SPSSX-L, send a message to [hidden email] (not to SPSSX-L), with no body text except the command. To leave the list, send the command SIGNOFF SPSSX-L For a list of commands to manage subscriptions, send the command INFO REFCARD ===================== To manage your subscription to SPSSX-L, send a message to [hidden email] (not to SPSSX-L), with no body text except the command. To leave the list, send the command SIGNOFF SPSSX-L For a list of commands to manage subscriptions, send the command INFO REFCARD |
Re: one way anova overall significant, but dunnett post hoc has no single significant group?
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In reply to this post by chengfoh
There is only one post-test comparison procedure that guarantees a significant result if you have a significant omnibus test and that is Scheffe. It, however, controls for all comparisons, including complex (ie. more than two means compared like (m1 + m2)/2 - m3 = 0).
Dr. Paul R. Swank, Professor Health Promotion and Behavioral Sciences School of Public Health University of Texas Health Science Center Houston -----Original Message----- From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of chengfoh Sent: Thursday, October 18, 2012 5:19 AM To: [hidden email] Subject: one way anova overall significant, but dunnett post hoc has no single significant group? Hi hope someone can really help, this confuse me alot I would like to compare the mean for 3 test groups to a control group fro different parameters (RBC, ALT, ALP....), so i ran a oneway ANOVA and then i conduct the post hoc using Dunnett's test..then i observed someting confusing me, whereby There were parameter whereby when ANOVA overall P value was significant, however dunnett test gave no no single group with significant value, how possible? Also, there were situations where overall ANOVA test was not significant, but posthoc (which i included sometime for test run) show some test groups with significant P value as compared to contorl... Wish someone came across with these can help me, many thanks Note: the number of subject in each group might differ, for example, each group can have 3/4 animals and is not throughouhly fixed -- View this message in context: http://spssx-discussion.1045642.n5.nabble.com/one-way-anova-overall-significant-but-dunnett-post-hoc-has-no-single-significant-group-tp5715709.html Sent from the SPSSX Discussion mailing list archive at Nabble.com. ===================== To manage your subscription to SPSSX-L, send a message to [hidden email] (not to SPSSX-L), with no body text except the command. To leave the list, send the command SIGNOFF SPSSX-L For a list of commands to manage subscriptions, send the command INFO REFCARD ===================== To manage your subscription to SPSSX-L, send a message to [hidden email] (not to SPSSX-L), with no body text except the command. To leave the list, send the command SIGNOFF SPSSX-L For a list of commands to manage subscriptions, send the command INFO REFCARD |
Re: one way anova overall significant, but dunnett post hoc has no single significant group?
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I've always thought of the Scheffe procedure in the opposite
direction. That is: It sets a high standard for the individual contrast, so that if there is *any* contrast that is significant, then the omnibus test has to be. (Basically, the contrast will account for the full Sum of Squares needed for the overall test and d.f.) -- Rich Ulrich > Date: Thu, 18 Oct 2012 17:22:33 -0500 > From: [hidden email] > Subject: Re: one way anova overall significant, but dunnett post hoc has no single significant group? > To: [hidden email] > > There is only one post-test comparison procedure that guarantees a significant result if you have a significant omnibus test and that is Scheffe. It, however, controls for all comparisons, including complex (ie. more than two means compared like (m1 + m2)/2 - m3 = 0). > > ... |
Re: one way anova overall significant, but dunnett post hoc has no single significant group?
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The way I recall Scheffe is that it pertains to the set of all possible linear combinations of means, not simply the very small subset of pairwise contrasts.
http://www.itl.nist.gov/div898/handbook/prc/section4/prc472.htm
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Re: one way anova overall significant, but dunnett post hoc has no single significant group?
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In reply to this post by Rich Ulrich
I wasn't complaining. I was just pointing to the
direction that has been useful to me. (Mostly, it has been useful in teaching people about post-hoc testing.) -- Rich Ulrich From: [hidden email] To: [hidden email]; [hidden email] Date: Thu, 18 Oct 2012 19:25:58 -0500 Subject: RE: one way anova overall significant, but dunnett post hoc has no single significant group? IIRC, the theorem is if and only if, that is, it does work both ways.
Dr. Paul R. Swank, Professor Health Promotion and Behavioral Sciences School of Public Health University of Texas Health Science Center Houston
From: Rich Ulrich [mailto:[hidden email]]
I've always thought of the Scheffe procedure in the opposite > Date: Thu, 18 Oct 2012 17:22:33 -0500 |
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Rich, Thanks for doing that; I appreciate your explanations on the list. –Steve (Pace University) From: SPSSX(r) Discussion [mailto:[hidden email]]
On Behalf Of Rich Ulrich I wasn't complaining. I was just pointing to the
From: [hidden email] IIRC, the theorem is if and only if, that is, it does work both ways. Dr. Paul R. Swank, Professor
Health Promotion and Behavioral Sciences School of Public Health University of Texas Health Science Center Houston From: Rich Ulrich [mailto:[hidden email]]
I've always thought of the Scheffe procedure in the opposite > Date: Thu, 18 Oct 2012 17:22:33 -0500 |
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